This invention relates to surface acoustic wave filters in amplitude modulation systems and, more particularly, to amplitude modulation broadcast transmitters using such filters.
In those television transmitters wherein conventional IF modulation is performed, inductive-capacitive (L-C) filters are utilized at IF so as to shape the signal and thereby meet the signal bandwidth and amplitude requirements imposed by the Federal Communications Commission. In addition thereto, all pass L-C or active phase equalizers are utilized either at baseband (video) frequency or IF to equalize the delay introduced by the L-C filters. The use of IF L-C filters for signal shaping requires that the filters be tuned at the factory, whereas the delay equalization at video frequencies is dependent upon the vestigial slope of the demodulator located in the receiver. This dependency introduces distortion at the receiver.
Another type of television broadcast transmitter is described in U.S. Pat. No. 3,955,155 issued on May 4, 1976, to RCA Corporation, based on the invention of William Behrend, entitled "Amplitude Modulation System." The system described therein achieves the required signal shaping by the use in the IF stage of a vestigial sideband filter (VSBF) followed by a delay equalizer. The sideband filter and equalizer is typically tuned in the factory thereby rendering filter replacement at the broadcast station difficult. In addition thereto, the characteristics of the filter change with time.
Surface acoustic wave (SAW) filters are quite flat in amplitude response and linear in phase. They have the fault, however, in that there are amplitude ripples and phase ripples caused by a multiple transit signal reflected from the receiving and sending ends that adds to the direct signal at the output. If the output wave is the sum of the direct wave and the multiple transit wave, the phase ripples and amplitude ripples are related and have the same period. The magnitude of the multiple transit signal and thus the magnitude of the phase and amplitude ripples are determined by the insertion loss of the filter. The amplitude ripples and the phase ripples cause echoes in the TV signal, producing distortion in the TV signal. This distortion exceeds that suitable for high quality TV broadcasting.